Abstract

OBJECTIVES

We sought to test the postulate that biventricular pacing diminishes the need for appropriate tachycardia therapy. We reviewed the frequency of therapy in patients, serving as their own controls, who were enrolled in the Ventak CHF (congestive heart failure) biventricular pacing study.

BACKGROUND

It is well established that both acute and chronic CHF contribute to the need for tachyarrhythmia therapy in recipients of an automatic implantable cardioverter defibrillator (ICD). Synchronized biventricular (BV) pacing is a new and promising therapy for symptomatic improvement of CHF in selected patients (low ejection fraction, intraventricular conduction delay). We postulate that this pacing therapy will diminish the need for tachyarrhythmia therapy.

METHODS

Participants in the Ventak CHF trial received a triple-chamber biventricular ICD with a transvenous right ventricular lead and a left ventricular (LV) lead placed via thoracotomy. Of 54 patients enrolled in the Ventak CHF trial, 32 could be analyzed, with each completing three blinded months programmed to BV VDD pacing and a second randomly assigned three-month period of no pacing.

RESULTS

Of the 32 patients, 13 (41%) received appropriate therapy for a ventricular tachyarrhythmia at least once in the six-month monitoring period postimplant. Five patients (16%) had at least one tachyarrhythmic episode while programmed to BV pacing, whereas 11 (34%) had at least one episode while programmed to no pacing. Three patients (9%) received therapy in both pacing periods, two with BV pacing only. The decrease in necessary tachycardia therapy during the BV pacing period was statistically significant (p = 0.035).

CONCLUSIONS

In patients with standard ICD indications who also have CHF, LV dysfunction, and an intraventricular conduction delay, ICD therapy is less common with BV pacing. The mechanism for this improvement is unclear but may be related to hemodynamic improvement in CHF. Although BV pacing does not obviate the need for an ICD, it does diminish the need for appropriate tachyarrhythmia therapy in selected patients.

Ventricular arrhythmias and resultant implantable cardioverter defibrillator (ICD) therapy are more common in patients with congestive heart failure (HF) (1). This correlation occurs both when an individual patient has an exacerbation of HF as well as across patient populations. Despite similar indications for an ICD, patients are more likely to receive an ICD shock if they have a lower ejection fraction or a higher New York Heart Association (NYHA) functional class (2,3). Conversely, treatment of HF is associated with a lower incidence of ICD therapy delivery.

Early data suggest that, in selected patients, biventricular (BV) pacing is an effective treatment for symptomatic HF—though this is still being validated (4,5). The mechanism involved in this treatment is still under study. Ever since its inception, it has been recognized that BV pacing for HF may be complementary to ICD therapy for life-threatening ventricular arrhythmias. However, it is possible that the two treatments may have synergistic benefits. Specifically, BV pacing therapy for HF may impact the need for ICD therapy for tachyarrhythmias, either as a proarrhythmic or antiarrhythmic factor. Our postulate is that electrical treatment of HF utilizing BV pacing will reduce the need for ICD therapy.

Methods

Trial design

Patients evaluated were all participants in the Ventak CHF trial. The study is an as-yet-unpublished blinded, randomized comparison of BV pacing with “no pacing” (see protocol below) in candidates for an ICD. Participants must have symptomatic HF (NYHA functional class II or greater), an ejection fraction below 0.35 and a QRS width of ≥120 ms (6). For this study, a transvenous system for left ventricular (LV) pacing was not available. All LV leads were placed via thoracotomy at a mid- or apical site on the anterior or lateral LV epicardium (see Fig. 1). Although study patients had an intraventricular conduction delay, no patient was enrolled who had a requirement for a permanent pacemaker or who had chronic atrial fibrillation.

Postero-anterior radiograph of a patient with a Ventak CHF ICD system. Note the standard dual-chamber transvenous leads as well as two (one redundant) unipolar epicardial leads in the mid-lateral left ventricle.

After obtaining informed consent, unipolar epicardial pacing leads were placed via thoracotomy and tunneled to a left subclavicular generator (Fig. 1). Traditional transvenous dual chamber ICD leads were placed via the left subclavian venous system and also attached to the generator, which was implanted either subcutaneously or submuscularly in the left chest similar to standard transvenous ICD implantation techniques (7).

Protocol

After a one-month postoperative recovery period, patients were randomized to receive a three-month period of either atrial synchronous ventricular pacing (VDD) or no pacing, which actually comprised BV VVI pacing at 40 ppm maintained for safety reasons. The pacing mode was blinded to the patient. A second three-month period followed with the pacemaker in the alternative therapy mode. At the conclusion of each treatment period, patients underwent ICD interrogation, including evaluation of episode frequency, stored electrograms, histograms, percent paced, and other parameters.

Statistical analysis

All tests were two-sided, and p values below 0.05 were considered significant. Continuous paired variables were compared with a paired Student t-test. Statistical tests were performed with Statview 5.0 for Windows (SAS Institute, Cary, North Carolina). Period effects, carryover effects, and other interactions between treatment and period were tested using the methodology described by Pocock (8). Descriptive variables are reported as mean ± standard deviation.

Results

Demographics

A total of 54 patients were evaluated. The mean age was 65 ± 10 years, with 74.1% men, and NYHA functional classes distributed as follows: class I, 0%; class II, 22%; class III, 65%; and class IV, 13%. The cardiac disease process included 70% with an ischemic cardiomyopathy, with the remainder nonischemic dilated cardiomyopathies subcategorized as idiopathic in 26%, hypertensive in 2%, and alcoholic in 2%. The type of interventricular conduction delay was left bundle branch block in 80%, right bundle branch block in 7%, and nonspecific in 13%. The primary indication for an ICD was symptomatic monomorphic ventricular tachycardia (VT) in 43%, nonsustained VT meeting the MADIT (Multicenter Automatic Defibrillator Implantation Trial) criteria in 30%, ventricular fibrillation in 15%, and polymorphic VT in 9%, with 4% unreported (9).

Patients for analysis

Nine patients died before completion of both arms of the protocol, and one other withdrew from the study. Two patients had fractures of the LV lead, which were not repaired, and they were thus excluded from analysis. Four patients have not completed the second phase of evaluation; six have incomplete data sets related to the second phase. Thus, from the original 54, there are 32 patients available for paired analysis. Episodes reported are combined for both shocks and antitachycardia pacing (ATP) because discrimination between the two modes of tachycardia termination was not available. However, antitachycardia schemes were not modified for either of the two periods studied. All episodes included had electrogram confirmation of a ventricular tachyarrhythmia.

Of the 32 completed patients, 13 (41%) received appropriate therapy for a ventricular tachyarrhythmia at least once in the six-month monitoring period postimplant. Five patients (16%) had at least one tachyarrhythmic episode while programmed to BV pacing, whereas 11 (34%) had at least one episode while programmed to no pacing (Table 1). Three patients (9%) received therapy in both pacing periods, two with BV pacing only. Thus, of those patients who received ICD therapy at any time, 8 of 13 (62%) therapies occurred during the no-pacing period only, two (15%) during BV pacing only, and three (23%) during both periods. For individual patients receiving antitachycardia therapy, the decrease in the ICD therapy noted in the BV pacing mode was suggestive but did not achieve statistical significance (p = 0.058), although it did for antitachycardia episodes (p = 0.035).

Study end points

There were 20 therapy episodes in the 32 patients (0.6 ± 2.1) during BV pacing, while there were 44 episodes (1.4 ± 3.5) in the same patient population during no pacing (Table 2). This difference was found to be statistically significant (p = 0.035). These findings persisted regardless of pacing order (no pacing or VDD pacing) of the three-month pacing periods. Period effect and treatment-period interaction effects were found to be nonsignificant with t-tests.

Number of Therapy Episodes in the 32 Data Pairs During Each of the 3-Month BV (Biventricular) Pacing and No-Pacing Periods

It is interesting that there was no difference in number of therapy episodes per patient for those who received therapy. Of the five receiving therapy in BV mode, there were 4.0 ± 4.1 antitachycardia episodes per patient versus 4.0 ± 5.1 episodes per patient in the 11 patients in the no-pacing mode. Thus, ICD therapy was less common in the BV pacing mode, but if therapy was needed, patients in the BV pacing mode required a similar number of treatment episodes as when therapy was required in the no-pacing mode.

Discussion

Potential mechanisms

In patients serving as their own controls, ICD therapy (both shocks and ATP) was less common with BV pacing than with no pacing (actually, pacing provided as a backup at 40 ppm). There are several potential mechanisms for this diminution in the need for tachyarrhythmia therapy: a decrease in ventricular conduction delays with BV pacing contributing to a decrease in macro-reentry; avoidance of pause-dependent tachyarrhythmias; and a decrease in plasma norepinephrine levels with BV pacing as well as other effects of pacing on the mechanism of arrhythmia induction (10). Research continues with respect to whether this diminution in ICD therapy with BV pacing is also observed with transvenous LV leads placed via the coronary sinus, as opposed to the epicardial active fixation leads of the present study (11).

Although it is not reported here, improvement in ventricular hemodynamic performance (and clinical HF symptoms) remains a probable contributing factor for the observed prevention of the need for shock therapy with BV pacing (5). As mentioned, several prior studies, both using patients as concurrent controls and matched trials, have shown that ICD therapy is less common when HF is clinically compensated (1–3). The mechanism for this observation is not well understood, but it probably includes a decrease in localized conduction delays as a result of diminished ventricular chamber sizes.

Biventricular pacing without icd backup

Although BV pacing may not obviate the need for ICD therapy, it appears to significantly diminish the number of appropriate ICD therapy episodes (ATP and shocks). It has been postulated that BV pacing can be applied without concomitant availability of ICD tachyarrhythmia therapy. We evaluated preoperative clinical parameters in an attempt to determine whether subsequent need for ICD therapy could be reliably anticipated. Unfortunately, we could not determine such predictive factors.

Study limitations

The Ventak CHF study was designed to determine the safety and efficacy of BV pacing in the treatment of HF in patients with indications for an ICD. In our analysis, we find that this therapy may limit the need for tachyarrhythmia treatment. However, it is an observed retrospective finding and thus may represent a coincidental or Type I error.

Although we postulate that improvement in HF contributes to the decreased need for tachyarrhythmia therapy, we have no proof of the direct relationship of these findings. Certainly, it is possible that other mechanisms may be responsible. The insertion of multiple epicardial leads could undoubtedly play a role in the generation or prevention of arrhythmias. For example, it has been shown that multisite pacing in the atrium diminishes the incidence of certain atrial arrhythmias (12). In addition, the current configuration of the ICD lead system precluded comparison of right ventricular pacing with the BV configuration, so the differences noted could be a function of dual-chamber right ventricular pacing alone rather than BV pacing. Finally, the study evaluated the potential benefit of BV pacing in a three-month interval beginning one month after implantation. The permanence of arrhythmia modification with BV pacing was not assessed. Despite the diminished antitachycardia therapy observed, the mortality remained high for the study participants. Presumably, this was related to the selection requirements of patients with severe symptomatic HF (78% NYHA classes III–IV).

Conclusions

In selected patients with standard ICD indications who also have HF and an intraventricular conduction delay, ICD therapy (shocks and ATP) is less common with BV pacing. The mechanism for this reduction of ventricular arrhythmias is unclear, although it is presumed to be caused, at least partly, by the improvement in LV performance achieved with atrial-synchronous BV pacing. Although BV pacing may not obviate the need for ICD therapy, it appears to significantly diminish the number of appropriate ICD therapy episodes (ATP and shocks). Further study of the mechanism and clinical significance of this interesting observation is necessary.

Footnotes

Presented in part at the 20th Annual Scientific Sessions of the North American Society of Pacing and Electrophysiology, Toronto, Canada, May 1999.

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